The present invention relates to a method for characterizing the modal power distribution of an optical signal launched from a laser source into a multimode optical fiber.
There is currently a great deal of interest in optical local area networks operating at speeds of a Gbps or more. An Ethernet standard for such transmission is now in place and will inevitably function to accelerate the use of high speeds optical LANs. In order to achieve these high bit rates, semiconductor lasers (for example, vertical cavity surface emitting lasers (VCSELs) or Fabry-Perot (FP) lasers) will be used as transmission sources. In the case of most optical LANs, however, multimode fiber will be used, due to both its ease of installation compared to single mode fiber and the fact that there exists a significantly large embedded base of multimode fiber.
Historically, multimode fiber has been used with LEDs as sources, not lasers; where LEDs are known to launch nearly equal power into every mode (defined as an xe2x80x9coverfilledxe2x80x9d launch). In the current application for use with lasers, however, the optical launches will be very far from overfilled, based on the characteristics of laser emission. In order to develop fast, reliable, low-cost systems, it is crucial to understand the behavior of multimode fiber under these restricted, laser-based launched conditions.
It has previously been discovered that restricting the launch in the laser mode does not always increase the bandwidth of a multimode fiber relative to the overfilled launch bandwidth, as might be intuitively expected. For most fibers and most restricted launches, a bandwidth gain is seen, but there are also cases where the bandwidth actually decreases. The latter is a concern in attempting to develop a set of transmission standards for a gigabit Ethernet (GbE). In particular, one source of bandwidth decrease has been found to be attributable to refractive index perturbations near the fiber axis that are common to all major multimode fiber manufacturing techniques, resulting in a standards requirement for xe2x80x9coffset launchesxe2x80x9d that avoids the center of the multimode fiber for certain applications.
The GbE standard is generally believed to be quite conservative. To close the gap between the standard and the optimal use of multimode fiber in LAN systems, accurate measurement tools are required. One such measurement tool is the measure of modal power distribution (MDP) of a source (i.e., laser) into a multimode fiber. The MPD essentially characterizes a launch, and is therefore clearly fundamental to understanding the behavior of a multimode fiber link. Also, many different optical sources for GbE are currently under development, so an accurate and robust measurement technique for evaluating the MPD induced by these sources is useful for both source development and selection.
One prior art technique for measuring MPD is based on calculations using ray optics, which becomes exact as the number of modes tends toward infinity. This technique, as described in detail in Telecommunications Industry Association (TIA)/Electronics Industries Association (EIA) Telecommunications System Bulletin, ITM-3, xe2x80x9cMode Power Distribution and Mode Transfer Function Measurementxe2x80x9d, dated Aug. 1995, requires a continuous wave (cw) measurement of the near-field intensity at the end of a length of fiber (which is generally accomplished with a CCD camera). For many applications, ray optics is a useful approach but for restricted launch situations, the approximation is not appropriate and the standard MPD reconstruction algorithm may yield incorrect results.
Another prior art technique again involves processing near-field intensity, but reconstructing the MPD using a modal, rather than a ray-optics, calculation This technique is discussed in detail in an article entitled xe2x80x9cModal Excitation of Optical Fibers: Estimating the Modal Power Distributionxe2x80x9d, TIA Draft Notes 2.2, J. S. Abbott, 1998. However, limitations in the near-field intensity data make the reconstruction problem of find the MPD difficult, and can thus limit the accuracy of the results in practice.
Thus, a need remains for properly characterizing the modal power distribution of an arbitrary laser source as launched into a multimode fiber.
The need remaining in the prior art is addressed by the present invention, which relates to a method for characterizing the modal power distribution of an optical signal launched from a laser source into a multimode optical fiber and, more particularly, to the use of a pulsed output from the source being tested as launched through a test fiber that will temporally separate the pulses. Additionally, an image of the test fiber endface may be obtained to provide both temporal and spatial data for reconstructing the MPD of the source.
In accordance with the present invention, the MPD measurement technique includes the steps of: (a) characterizing a predetermined test fiber (multimode) with a differential modal delay (DMD) measurement by scanning a single mode fiber across the input end of the test fiber and launching a short pulse at each position; (b) characterizing a launch condition resulting from a specified source under test (a reverse DMD measurement) using short pulses launched from the source with a single mode fiber scanned across the output end of the fiber; and (c) applying a reconstruction algorithm to recover the launched modal power from the data obtained in the first two steps.
In an alternative process of the present invention, the step of characterizing the reverse DMD may be omitted if the mode groups of the test fiber have sufficiently widely separated group velocities, the test fiber is long enough, and the source is capable of launching sufficiently short pulses at high enough powers.
Other and further features of the technique of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.